The devices used to check attitude are therefore based on an appropriate measuring system, which measures the values upon which the characteristic attitude parameters depend. The measured values are then transmitted to a processor which uses known mathematical/geometrical algorithms to calculate the characteristic angles of the wheels, and other attitude parameters if required, compares them with the correct values which are stored in its memory, relative to the vehicle model being worked on, and finally calculates the corrections necessary to restore the values to the permitted values, displaying the corrections on a monitor, and also printing a paper copy if required.
The detection systems at present in use can be grouped into two categories: those which effect measurements through direct contact with the wheels, and those which effect measurement without direct contact with the wheels.
Detection systems belonging to the first category generally comprise a plurality of operating heads, each of which can be attached to a relative wheel of the vehicle, and is provided with appropriate mechanical or electronic angle transducers, which detect its position and orientation. Data detected by the operating heads can be transmitted to the processor via cable or through a wireless system, for example via radio or via infrared light. In detection systems belonging to the second category, the operating heads are replaced by measuring instruments which are generally of the opto-electric type, based on acquiring and subsequently processing images of the wheel tracked by one or more video cameras.
In the images taken by the video cameras, the opto-electronic measuring instruments detect the positions of appropriate targets which are associated to the wheel, in such as way as to determine the equation of the plane or of the axis of rotation of the wheel in a predetermined frame of reference installed in the measuring instrument.
The targets can be appropriately shaped physical bodies, fixed to the wheels of the vehicle before performing the measurement, or they can be produced by projecting laser or structured light beams onto the wheels of the vehicle, which beams can give rise to simple luminous lines which radially cross the wheels, or more complex and appropriately coded patterns.
Opto-electronic measuring instruments also exist which do not use any type of encoded target, since in the images tracked by the video cameras they identify the position of lines actually on the wheels themselves, such as for example the separation border between the rim and the tyre. An instrument of this type is described in European patent application EP0895056 in the name of the same Applicant.
Whatever detection technique is used, opto-electronic measuring instruments are usually installed on appropriate fixed structures, from which they locate the targets associated to the wheels of the vehicle, or alternatively, they, are installed on portable structures which can be moved and positioned as desired by an operator, so as to situate the measuring instruments in appropriate positions in relation to the wheels of the vehicle to be measured. Intermediate solutions are also known, in which mobile measuring instruments are installed on board fixed structures, so that their relative position can be varied on the basis of the dimensions of the vehicle to be measured. For example, in the above mentioned application for European patent EP895056, the measuring instruments are slidingly installed on board a vehicle-lifting platform.
U.S. Pat. No. 6,456,372 discloses a device that determines, along a vehicle assembly line, the orientation of the steerable wheels with respect to the non steerable wheels of a motor vehicle by means of a plurality of cameras mounted on movable. Units that may follow a path defined by a horizontal rail that allows movement of the measuring devices only along one direction. The steerable wheels are thus set in a straight ahead position for connection of a steering wheel. Laser devices measure the relative angles of the front and rear wheel hubs as the vehicle is carried along the assembly line.
WO 2006/052684 discloses rails for making the path for a measuring unit having at least two cameras that are able to visualize the bottom of the vehicle. In such device there is also a horizontal cross member fixed to the upper ends of two vertical support members that carries, along a mono-dimensional travel path, a front-end measuring unit. The cameras on this unit may pivot along a horizontal axis. Combination of the measurement taken by such device with the help of suitable target may allow an operator to determine the deformation of the frame of the car due to an accident in order to decide for appropriate repair procedure.
U.S. Pat. No. 6,072,433 discloses a system and method using a plurality of sending and receiving antennas for determining the alignment in space of a formation of moving objects, such has spacecrafts or artificial satellites.
The solutions which are in use at present and are briefly illustrated above are however not free of drawbacks.
In particular, fixed and semi-fixed structures have the drawback of being bulky, and inside the workshop require a relatively large area to be dedicated exclusively to adjusting vehicle attitude.
Mobile structures on the other hand have the drawback that before performing measurements they must be appropriately positioned around the vehicle by the operator, with possible positioning errors and increased working times, and must also be stored away when operations are finished.